Polymeric electrostatic toner particles can be prepared by a process frequently referred to as “limited coalescence”. In this process, polymer particles having a narrow size distribution are obtained by forming a solution of a polymer in a solvent that is immiscible with water, dispersing the solution so formed in an aqueous medium containing a solid colloidal stabilizer and removing the solvent by evaporation. The resultant particles are then isolated, washed and dried.
In the practice of this technique, toner particles are prepared from any type of polymer that is soluble in a solvent that is immiscible with water. Thus, the size and size distribution of the resulting particles can be predetermined and controlled by the relative quantities of the particular polymer employed, the solvent, the quantity and size of the water insoluble solid particulate suspension stabilizer, typically silica or latex, and the size to which the solvent-polymer droplets are reduced by agitation.
Limited coalescence techniques of this type have been described in numerous patents pertaining to the preparation of electrostatic toner particles because such techniques typically result in the formation of toner particles having a substantially uniform size distribution. Representative limited coalescence processes employed in toner preparation are described in U.S. Pat. Nos. 4,833,060 and 4,965,131 to Nair et al.
As with other emulsion related techniques, toner particles prepared with the limited coalescence process generally tend to be spherical in shape. On the other hand, conventional toners produced with the pulverizing technique, or commonly referred to as “ground toners”, are of irregular surface morphology. There have been efforts to manufacture irregular shaped toner particles with the limited coalescence process. U.S. Pat. No. 5,283,151 is representative of earlier work in this field and describes the use of carnauba wax to achieve similar toner morphology. The method comprises the steps of dissolving carnauba wax in ethyl acetate heated to a temperature of at least 75° C. and cooling the solution, so resulting in the precipitation of the wax in the form of very fine needles a few microns in length; recovering the wax needles and mixing them with a polymer material, a solvent and optionally a pigment and a charge control agent to form an organic phase; dispersing the organic phase in an aqueous phase comprising a particulate stabilizer and homogenizing the mixture; evaporating the solvent and washing and drying the resultant product.
Unfortunately, this technique requires the use of elevated temperature to dissolve the wax in the solvent and cooling the solution to precipitate the wax. The wax does not stay in solution of ethyl acetate at ambient temperature and as a result it is very difficult to scale up using this methodology.
The shapes of the toner particles are important to the performance of an electrostatic toner such as transfer and cleaning properties. Thus, for example, the transfer and cleaning efficiency of toner particles have been found to improve as the sphericity of the particles are reduced. Thus far, workers in the art have long sought to modify the shape of the evaporative limited coalescence type toner particles by means other than the choice of pigment, binder, or charge agent. Several approaches to modifying the shape of the toner particles have been disclosed in the art, and most involve the use of external chemical agents introduced into the formulation and process.
U.S. Pat. No. 5,968,702 discloses a method of employing commercially available SOLSPERSE® hyperdispersents, such as SOLSPERSE® 24000 or 20000, in the organic phase in the evaporative limited coalescence process. Toner particles of controlled morphology can be obtained.
However, it has been found that the use of these hyperdispersants may lead to toner particles of unstable charge. Particularly, when negative charging is desired, toner particles obtained using SOLSPERSE® 24000 as the shape control agent may exhibit positive or erratic tribo-charging. Unpredictable and unstable charging behavior of a toner is unacceptable in electrophotography using dry toner powder, where a uniform and stable charge of the toner particles is required.
U.S. Pat. No. 6,380,297 describes the use of commercially available surfactants in controlling toner shape. These shape-modifying agents are used after the homogenization step, and therefore an additional step of introducing these agents to the homogenized emulsion is necessary.
U.S. Pat. No. 6,416,921 to Wilson et al. describes the use of quaternary ammonium tetraphenylborate salts and a polymeric phosphonium salt for controlling morphology of the toner particles. These polymeric materials generally yield irregularly shaped toner particles while maintaining acceptable charging behavior.
U.S. Pat. No. 6,482,562 to Ezenyilimba et al. describes the use of flocculating agents as shape control addenda in the ELC process, which are typically introduced into the aqueous phase either before or after the homogenization step.
Furthermore, US 20080145779A1 entitled “Toner Particles of Controlled Surface Morphology and Method of Preparation” describes the use of certain metal carbamates that are useful to control sphericity and US 20080145780A1 entitled “Toner Particles of Controlled Morphology” describes the use of specific salts to control sphericity.
The above mentioned methods of manufacturing non-spherical toner particles all rely on the action of surface active agents during the solvent removal step. Not only do they generally require more complex manufacturing processes, often the use of these materials can cause undesired performance issues such as toner charging, dusting, and melt rheology variations.